Multi-stage combustors are used particularly in lean burn fuel systems of gas turbine engines to reduce unwanted emissions while maintaining thermal efficiency and flame stability. For example, duplex fuel injectors have pilot and mains fuel manifolds feeding pilot and mains discharge orifices of the injectors. At low power conditions only the pilot stage is activated, while at higher power conditions both pilot and mains stages are activated. The fuel for the manifolds typically derives from a pumped and metered supply. A splitter valve can then be provided to selectively split the metered supply between the manifolds as required for a given staging.
A typical annular combustor has a circumferential arrangement of fuel injectors, each associated with respective pilot and mains feeds extending from the circumferentially extending pilot and mains manifolds. Each injector generally has a nozzle forming the discharge orifices which discharge fuel into the combustion chamber of the combustor, a feed arm for the transport of fuel to the nozzle, and a head at the outside of the combustor at which the pilot and mains feeds enter the feed arm. Within the injectors, a check valve, known as a fuel flow scheduling valve (FFSV), is typically associated with each feed in order to prevent combustion chamber gases entering the fuel system. The FFSVs also prevent fuel flow into the injector nozzle when the supply pressure is less than the cracking pressure.
Multi-stage combustors may have further stages and/or manifolds. For example, the pilot manifold may be split into two manifolds for lean blow-out prevention.
During pilot-only operation, the splitter valve directs fuel for burning flows only through the pilot fuel circuit (i.e. pilot manifold and feeds). It is therefore conventional to control temperatures in the stagnant (i.e. mains) fuel circuit to prevent coking due to heat pick up from the hot engine casing. One known approach, for example, is to provide a separate recirculation manifold which is used to keep the fuel in the mains manifold cool when it is deselected. It does this by keeping the fuel in the mains manifold moving, although a cooling flow also has to be maintained in the recirculation manifold during mains operation to avoid coking.
A problem associated with this approach is that blockage may occur in the recirculation line. The consequence of such a failure is dependent on the location of the blockage. For example, if the blockage occurs on the recirculation downstream of the injectors, the result can be an increased pressure in the recirculation line which opens the mains FFSVs, potentially causing hot streaks and, as a consequence, turbine damage. If the blockage occurs on the recirculation line upstream of the injectors, the result can be a loss of cooling flow and/or pressure in the recirculation line at the injectors, potentially resulting in combustion gases leaking past the mains FFSVs and thence to the low pressure side fuel system of the system via the exit from recirculation line. This can lead to damage and/or failure within the fuel system.